The present invention relates to a galvanic gold alloying bath which contains 1 to 15 g/liter gold as potassium gold (I) cyanide, 5 to 50 g/liter copper as potassium copper (I) cyanide, 0.05 to 5 g/liter silver as potassium silver (I) cyanide, free alkali cyanide, dipotassium hydrogenphosphate as well as a selenium compound and exhibits a pH of 8.5 to 11.
The galvanic plating and deposition of gold alloys has achieved special significance for decorative and industrial purposes. The soft, light yellow, matte layers of pure gold can be varied in many ways in respect of their physical properties, for example, luster or brightness, hardness, wear resistance or color, by means of the coplating of other metals. A considerable part of the plated, gold-alloy coatings is constituted by 14-18 carat, yellow or rose-colored gold coatings containing, in addition to copper as an alloying metal, a metal which imparts a white color such as cadmium, silver or zinc in order to brighten the red tone caused by the copper.
Such coatings are used for example in the jewelry trade and in the eyeglass industry, where double layers have been largely replaced by galvanic coatings. However, such coatings are also used in electronic technology applications if no low contact resistance is necessary, such as for example in the case of slip-ring contacts and rotary contacts.
The galvanic plating of gold/copper/silver alloy coatings poses considerable difficulties on account of the potential position of the metals in the electrolyte. Ways to solve these difficulties have not been altogether unsatisfactory up to the present. The basis for the common plating system are aqueous solutions of the cyanocomplexes of the three metals. In the alkaline range, in which these baths are only stable, the potential of silver is considerably more electropositive than that of gold and copper. This means that silver is preferentially plated out and therefore only coatings with a whitish-yellow or greenish-yellow color, depending on the silver content, are obtained. An attempt was therefore made in DE-PS 801,312 to shift the potential of silver to less electropositive, or less noble, values by using as low a pH as possible. However, at the indicated pH of 7, the free alkali cyanide required to stabilize the cyanocomplexes of the three metals is no longer stable. Furthermore, the concentration changes constantly which results in a constant changing in the composition of the plated alloy. Other prior art attempts, such as DD-PS 59022, likewise are carried out in the neutral range and efforts to obtain lustrous layers by means of using alternating current at a low concentration of free alkali cyanide frequently results in practice in obtaining coatings with an uneven color.
In alkaline baths, the formation of luster or brightness in the coatings can also be achieved with chemical compounds. According to DE-PS 750,185, selenium compounds or tellurium compounds impart luster in the plating of silver coatings or copper coatings from an alkaline cyanide bath.